FOREWORD
0.
3
.1971
Indian Standard
SPECIFICATION FOR PORCELAIN INSULATORS FOR OVERHEAD POWER LINES WITH A NOMINAL VOLTAGE GREATER THAN 1 000 V
( Second Revision )
0. 0. c) To prescribe test methods. †Specification for insulator fittings for overhead power lines of 3. and d) To prescribe acceptance criteria. which includes all the amendments issued to date to the first revision.IS : 731 . This second revision has been undertaken with a view to rationalizing the classification of tests.
*Specification for insulator fittings for overhead power lines of 3. after the draft finalized by the Electrical Insulators and Accessories Sectional Committee had been approved by the Electrotechnical Division Council.1 This Indian Standard (Second Revision) was adopted by the Indian Standards Institution on 18 November 1971.3 kV and above Part I General requirements and tests ( first revision ). b) To define insulator characteristics and to prescribe the conditions under which the specified values of these characteristics shall be verified.2 This standard has been prepared to achieve the following objects: a) To define the terms used. the classification of the insulators according to their construction has been incorporated and the requirements and methods of tests have been modified to keep the standard in line with the latest developments taking place in insulator technology.4 The fittings normally used with overhead porcelain insulators are covered in IS : 2486 (Part I)-1971* and IS : 2486 (Part II)-1963†.3 This standard was originally issued as a tentative specification for porcelain insulators in 1956. It was first revised in 1963 to make it more comprehensive and up to date. In this revision.3 kV and above Part II Dimensional requirements. 0. 0.

0 For the purpose of this standard. British Standards Institution.
1.5 In the preparation of this standard. expressing the result of a test.
NOTE 1 — The specification applies to string insulator units.7 For the purpose of deciding whether a particular requirement of this standard is complied with.1971
0. SCOPE
1. Unless otherwise stated. 2. 5 (July 1987) and Amendment No. Part II Requirements. shall be rounded off in accordance with IS : 2-1960*. It does not apply to insulators forming part of electrical apparatus or to parts used in their construction. the following definitions shall apply. to insulator strings consisting of one-string insulator unit or an assembly of such units. and to rigid over head line insulators.
*Rules for rounding off numerical values ( revised ). 6 (January 1993). The specification applies to insulators of the same type when used in sub-stations. for string insulator unit with ball and socket couplings the term includes the locking device (security clip) and for units with clevis and tonque coupling it includes the pin and other parts necessary for coupling. Amendment 5 takes into account the sequence of carrying out acceptance tests and introduction of double plan of sampling procedure. NOTE 2 — This specification applies to insulators for use on ac overhead lines and may be regarded as a provisional specification for insulators for use on dc over-head lines.
2. British Standards Institution.6 incorporates Amendment No. observed or calculated. 0. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard. TERMINOLOGY
2. Draft BS 137 : Part II : 1970 Insulators of ceramic material or glass for overhead lines with a nominal voltage greater than 1 000 V. 0. assistance has been derived from the following publications: IEC Pub 274 Tests on insulators of ceramic material or glass for overhead lines with a nominal voltage greater than 1 000 V.6 This edition 3.
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. BS 137 : Part I : 1970 Insulator of ceramic material or glass for overhead lines with a nominal voltage greater than 1 000 V.1 String Insulator Unit — An insulator consisting of a porcelain part with the necessary interconnecting metal-work to enable it to be flexibly attached to other string insulator units.1 This standard applies to the porcelain insulators for ac overhead power lines suitable for nominal system voltage greater than 1 000 volts and a frequency not greater than 100 Hz. International Electrotechnical Commission. Side bar indicates modification of the text as the result of incorporation of the amendments.IS : 731 . the final value. Part I Test.

2.3 without flashover or puncture.4 Shell — A single insulating member without cement or other connecting devices.7 Puncture — A disruptive discharge passing through the solid insulating parts of an insulator. 2. 2.5 Lot — All the insulators of the same type and design manufactured under similar conditions of production.3.2. Unless otherwise stated. 2. complete with fittings for flexible attachment to a supporting structure and to secure a line conductor or conductors in tension.
2.2 Line Post Insulator — A rigid insulator consisting of a porcelain part permanently secured in a metal base and intended to be mounted rigidly on a supporting structure by means of a stud attached to the base.
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.2 Insulator String — One or more string insulator units intended to give flexible support to an overhead line. this term includes the stud.2 Tension Insulator Set — One insulator string. The term ‘set’ includes all other metal parts and accessories as required in service.1 Pin Insulator — A rigid insulator consisting of a single piece of porcelain or of two or more porcelain components permanently connected together. 2.
NOTE — A fragment breaking away from the rim of a shed or damage to the insulator due to the heat of a surface discharge is not considered a puncture.1 Suspension Insulator Set — One insulator string or two or more strings suitably connected together.8 Dry Impulse Withstand Voltage — The specified impulse voltage which the insulator shall withstand. complete with fittings for flexible attachment to a supporting structure and to carry a line conductor or conductors at its lower end. this term excludes the insulator pin. Unless otherwise stated. a lot may consist of the whole or part of the quantity ordered. connecting those parts which normally have the operating voltages between them. 2. offered for acceptance. under the conditions specified in 10.3 Rigid Insulator — An insulator intended to give rigid support to an overhead line and to be stressed mainly by bending and compressive loads. The term ‘set’ includes all other metal parts and accessories as required in service. 2. 2.1971
2. and intended to be mounted rigidly on a supporting structure by an insulator pin passing up inside the insulator. or two or more strings suitably connected together. The insulator string is intended to be stressed only in tension.2.3.IS : 731 . 2.6 Flashover — A disruptive discharge external to the insulator.

15 Creepage Distance (of an Insulator) — The shortest distance or the sum of the shortest distances along the contours of the external surfaces of the porcelain insulating parts of the insulator between those parts which normally have the operating voltage between them.7.10. but may be as low as 104 ohms.13 Mechanical Failing Load — The maximum load which can be reached when a string insulator unit or a rigid insulator is tested under the conditions prescribed in 10.16 Highest Voltage of a System — The highest rms line-to-line voltage which can be sustained under normal operating conditions at any time and at any point on the system. If high-resistance coatings are applied to parts of the insulator.10 Wet Power-Frequency Withstand Voltage — The specified power-frequency voltage which the insulator shall withstand (wet) under the conditions prescribed in 10. 2.15.3 has a 50 percent probability of producing a flashover on the insulator. the questions of surface resistivity and creepage distance should be subject to agreement between the user and the manufacturer. 2.9 Fifty-Percent Dry Impulse Flashover Voltage — The impulse voltage which.11 Wet Power-Frequency Flashover Voltage — The arithmetic mean value of the measured power-frequency voltages which cause flashover of the insulator under the conditions prescribed in 10. 1 ). 2.
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.1 Protected Creepage Distance — That part of the creepage distance on the illuminated side of the insulator which would lie in shadow if light were projected on to the insulator in a direction at 90° to the longitudinal axis of the insulator ( see Fig. under the conditions prescribed in 10. 2.
2.4 for the specified time (one minute) without flashover or puncture.8. NOTE 2 — If high-resistance coatings are applied to the whole surface of the insulator (the so-called stabilized insulator). causes puncture. A distance over a cement surface shall not be considered as forming part of the creepage distance.12 Electromechanical Failing Load — The maximum load which can be reached when a string insulator unit is tested under the conditions prescribed in 10.4.
NOTE 1 — The surface resistivity of such high-resistance coatings is usually about 105 ohms. such coatings shall be considered effective creepage surfaces and the distance over them is included in the creepage distance ( see Fig. under the conditions prescribed in 10.1971
2. 2. 1 ). 2.IS : 731 .14 Puncture Voltage (of a String Insulator Unit or a Rigid Insulator) — The voltage which. 2. It excludes temporary voltage variations due to fault conditions and the sudden disconnection of large loads.

1
INSULATOR CREEPAGE DISTANCES (CREEPAGE DISTANCE SHOWN DOTTED)
.7 IS : 731 .
FIG.1971
NOTE — These sketches are only illustrations for total creepage and protected creepage distances and the same should not be mistaken for standard configuration.

IS : 731 .19 Type Tests — Tests carried out to prove conformity with the specification. using for the rotating machines the subtransient reactances.18. for a given system layout.
2. NOTE 3 — This factor is used for the choice of the insulation level of the equipment to be installed at a selected location. These are intended to prove the general qualities and design of a given type of insulator. REFERENCE ATMOSPHERIC CONDITIONS
3. expressed as a percentage. is the ratio.21 Routine Tests — Tests carried out on each insulator to check requirements which are likely to vary during production.18 Earthed Neutral System — A system in which the neutral is connected to earth.
3.1971
2. 2. either solidly. 2.18. independently of the actual operating values of the voltage at that location. The practical rule given in the Note under 2.
2. to the line-to-line rms power-frequency voltage which would be obtained at the selected location with the fault removed. NOTE 2 — The factors of earthing are calculated from the phase-sequence impedance components of the system as viewed from the selected location.17 Factor of Earthing — The factor of earthing at a selected location of a three-phase system (generally the point of installation of an equipment).
NOTE — This condition is obtained approximately when.1 A three-phase system with effectively earthed neutral at a given location is a system characterized by a factor of earthing at this point which does not exceed 80 percent. of the highest rms line-to-earth power-frequency voltage on a sound phase at the selected location during a fault to earth (affecting one or more phases at any point).1 results from such a calculation. the ratio of zero-sequence reactance to the positive-sequence reactance is less than three and the ratio of zero-sequence resistance to positive-sequence reactance is less than one. or through a resistance or reactance of low enough value to reduce materially transient oscillations and to give a current sufficient for selective earth fault protection.
NOTE 1 — This factor is a pure numerical ratio and characterizes in general terms the earthing conditions of a system as viewed from the selected location.20 Acceptance Tests — Tests carried out on samples taken from the lot for the purpose of acceptance of the lot. 2.1 Reference atmospheric conditions at which insulator characteristics shall be expressed for the purpose of comparison shall be as given below: Ambient temperature Barometric pressure 20°C 1 013 millibars
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. for all system configurations. particularly in the case of systems in which the highest voltage for equipment is 100 kV or more.

GENERAL REQUIREMENTS 4. The glaze shall cover all the porcelain parts of the insulator except those areas which serve as supports during firing or are left unglazed for the purpose of assembly.1 The porcelain shall be sound.1971
Absolute humidity 11 g of water per cubic metre corresponding to 63 percent relative humidity at 20°C. 4. These conditions will replace those specified above when the corresponding test voltage values and correction factors are available. free from defects. The barometric pressure. and its thickness shall be as uniform as possible.
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4. and proper care shall be taken to locate the individual parts correctly during cementing. test may be carried out under conditions naturally obtaining at the time of the test. air temperature and humidity shall be recorded for the purpose of corrections.
NOTE 1 — A pressure of 1 013 m bar is equivalent to a pressure of 760 mm of mercury at 0°C. the glaze shall be brown in colour.80 × 10 t ) 760 NOTE 2 — The Indian Standard reference temperature of 27°C and corresponding humidity have not been specified because of the non-availability of the test voltage values and correction factors for these conditions.
3. the atmospheric pressure in millibars is:
–4 1 013h p = -----------------.4 Cement used in the construction of the insulator shall not cause fracture by expansion or loosening by contraction. thoroughly vitrified and smoothly glazed. If the height of the barometer is h mm Hg and the temperature of the mercury is t°C.IS : 731 . a temperature of 27 ± 2°C and relative humidity of 65 ± 2 percent).2 Unless otherwise specified. The cement shall not give rise to chemical reaction with metal fittings. The porcelain shall not engage directly with hard metal. Corrections of test voltages for atmospheric conditions is given in Appendix A.2 Tests for the purpose of this standard shall preferably be carried out under conditions of temperature and humidity specified in IS : 196-1966* (that is. 4. 5.3 The design of the insulator shall be such that stresses due to expansion and contraction in any part of the insulator shall not lead to deterioration. CLASSIFICATION 5. When this is not possible.1 Overhead line insulators are divided into two types according to their construction: Type A — An insulator or an insulator unit in which the length of the shortest puncture path through solid insulating material is at least
*Atmospheric conditions for testing. 4.( 1 – 1. and at the prevailing atmospheric pressure.

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.1. and d) Country of manufacture.2 Insulators may also be marked with the ISI Certification Mark.
8. The loads shall be transverse in the case of pin and line post insulators and axial in the case of string insulator units.
6. BASIC INSULATION LEVELS
6. The Standard Mark on products covered by an Indian Standard conveys the assurance that they have been produced to comply with the requirements of that standard under a well defined system of inspection. Type B — An insulator or an insulator unit in which the length of the shortest puncture path through solid insulating material is less than half the length of the shortest flashover path through air outside the insulator.IS : 731 .2 In this standard. MARKING
8.1971
equal to half the length of the shortest flashover path through air outside the insulator. Details of conditions under which a licence for the use of the Standard Mark may be granted to manufacturers or producers may be obtained from the Bureau of Indian Standards.3 The withstand and flashover voltages are referred to the reference atmospheric conditions. c) Minimum failing load in newtons. b) Month and year of manufacture. 6.
NOTE — The use of the Standard Mark is governed by the provisions of the Bureau of Indian Standards Act. power-frequency voltages are expressed as peak values divided by 2 and impulse voltages are expressed as peak values.1 The basic insulation levels of the insulators shall be as given in Table 1A and Table 1B. 8.1 Markings on porcelain shall be printed and shall be applied before firing. testing and quality control which is devised and supervised by BIS and operated by the producer. 1986 and the Rules and Regulations made there-under. Standard marked products are also continuously checked by BIS for conformity to that standard as a further safeguard. 6. MECHANICAL LOADS
7.1 Each insulator shall be legibly and indelibly marked to show the following: a) Name or trade-mark of the manufacturer.1 The insulators shall be suitable for the minimum failing loads specified in Table 2. 8.
NOTE — Type A insulators are of solid core type.
7.

1971
10. c) Electrical routine test (for Type B string insulators and rigid insulators (10.3.
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.3 Five such impulse voltage waves shall be applied to the insulator. The insulator shall be considered to have passed this test only if during this new series of tests there is no flashover or puncture. A power-frequency test voltage of the specified value given in Table 1A or Table 1B shall be applied in accordance with Appendix B and maintained at this value for five minutes. b) Mechanical routine test (for string insulator units only) (10.2/50 impulse wave of peak value equal to the specified value of the impulse withstand voltage corrected for atmospheric conditions in accordance with Appendix A. If only one flashover occurs.1 The insulator shall be tested dry under the conditions prescribed in Appendix B. 10.
10. the 50 percent impulse flashover voltage for positive and negative polarities may be determined by a suitable procedure [ see IS : 2071 (Part II)-1974* ]. During this time. it shall suffice to test with that polarity.3.2 The impulse generator shall be adjusted to produce a standard 1.5 To provide information when specially requested.13). precise forms of tests cannot yet be adopted as standard. 10. the insulator shall be considered to have passed the test.15). 10.
NOTE — Attention is drawn to the fact that ultrasonic tests may be used on Type A insulator to detect internal flaws and cracks. However.3 Impulse Voltage Withstand Test 10.3. the insulator shall be considered to have failed to comply with the standard. therefore.14).4 The insulator shall be capable of passing the impulse voltage withstand test with voltages of both positive and negative polarity.3.IS : 731 . observations shall be made and there shall be no sign of visible corona. a new series of ten impulse waves shall be applied. If during the application of these five waves puncture occurs or if there is more than one flashover. when it is evident which polarity will give the lower breakdown voltage.2 Visible Discharge Test — The test room shall be darkened and a period of five minutes shall be allowed for the observer to become accustomed to darkness. If there is no flashover or puncture.3.3 Routine Tests — The following tests shall be carried out as routine tests: a) Visual examination (10. 10.1.
*Methods of high voltage testing: Part II Test Procedures ( first revision ). Experience is necessary to interpret results of ultrasonic tests and. 10.

1 The insulator shall be arranged as prescribed in Appendix B. the insulator shall be exposed to the artificial rain produced in accordance with 3.2 The insulator shall not be damaged by these tests.5.2 Before the commencement of the test.4.5 To provide information when specially requested.3 shall be applied and then increased gradually to reach the test voltage in a time not less than five seconds.1971
10. a tolerance of: ± (0.4. particularly as regards any dimensions to which special tolerances apply (for example.3 The test voltage to be applied to the insulator shall be the specified value of the wet power-frequency withstand voltage adjusted for atmospheric conditions at the time of test ( see Appendix A ).4.4. 10. spacing) and details affecting interchangeability (for example.3 of IS : 2071 (Part I)-1974* for at least one minute before application of voltage and then throughout the test.5.4.IS : 731 . The flashover voltage shall be the arithmetic mean of five consecutive readings and the value after correction to standard atmospheric conditions ( see Appendix A ) shall be recorded. ball and socket dimensions).4 Wet Power-Frequency Voltage Withstand Test 10. Unless otherwise specified. The insulator shall not flashover or puncture during the application of the test voltage.03 d + 6) mm when d > 300 mm
*Methods of high voltage testing: Part I General definitions and test requirements ( first revision ).5 Verification of Dimensions — It shall be verified that the insulator unit is in accordance with the relevant drawings. 10. 10.3. but slight marks on the surface of the insulating parts or chipping of the cement or other material used for assembly shall be permitted.1 The impulse flashover voltages to be recorded shall be the positive and negative 50 percent impulse flashover voltage as measured above and corrected in accordance with Appendix A.3. The test voltage shall be maintained at this value for one minute [ see IS : 2071 (Part II)-1974† ]. the wet flashover voltage of the insulator may be determined by increasing the voltage gradually from about 75 percent of the wet withstand voltage to reach the flashover voltage in not less than five seconds.4 A voltage of about 75 percent of the test voltage as determined in 10.4. †Methods of high voltage testing: Part II Test procedures ( first revision ). 10. 10.04 d + 1. 10.
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.5) mm when d ≤ 300 mm ± (0. 10.

The quantity of water in the test tanks shall be sufficiently large for the immersion of the insulators so as not to cause a temperature variation of more than ± 5°C in the water. prescribed in 10.3) mm shall. For this purpose. the tests. without being placed in an intermediate container.03 S + 0.14. without being placed in an intermediate container. in a water bath maintained at a temperature of 70°C higher than that of the cold bath used in the rest of the test and left submerged for 15 minutes. be applied to the spacing. They shall then be withdrawn and quickly and completely immersed. Type B insulator shall be subjected for one minute to the power-frequency test prescribed in 10.
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.6 may be too severe and a test of reduced severity may then be applied by agreement between manufacturer and purchaser. the tolerance formula: ± (0. very large rigid insulators or string insulator units shall be considered as those having one of the following dimensions: L greater than 120 cm L = length of insulator D2 L greater than 80 000 cm2 D = greatest external diameter f greater than 2. The insulators shall withstand the appropriate test without cracking or puncture or mechanical breakage. in the cold water bath where they shall remain for 15 minutes.1971
is allowed on all dimensions for which special tolerances do not apply ( d being the dimension in millimetres ). the insulator shall be examined to verify the that they have not cracked. The time taken to transfer from either bath to the other shall be as short as possible and never exceed 30 seconds. *Specification for large hollow porcelains for use in electrical installation. where S is the spacing in millimetres. Type A insulator shall then be subjected to the mechanical test prescribed in 10. unless otherwise specified.
NOTE — Pending the publication of Indian Standard covering the spacing of string insulator units. This heating and cooling cycle shall be performed three times in succession.5 cm f = the greatest thickness defined by the diameter of the greatest circle which can be inscribed within the outline of a section through the axis of the hollow insulator d greater than 9 cm d = core diameter for solid core insulator NOTE 2 — The restriction against using an intermediate container does not exclude the use of a wire mesh basket having a low thermal capacity and giving free access for the water. NOTE 3 — Also see IS : 5621-1970*.IS : 731 . On completion of the third cold cycle. shall be quickly and completely immersed.
NOTE 1 — For very large rigid insulators or string insulator units.15.6 Temperature Cycle Test — Insulators with their integral parts.
10. if any.

1971
10. 10.8.1 This test shall be applied to string insulator units of such types where electrical discharge will serve to indicate mechanical failure.3 The load shall be gradually increased from a value of 75 percent of the specified mechanical failing load until the specified mechanical failing load is reached.8 Mechanical Failing Load Test 10. 10. 10.7.IS : 731 .1 String insulator units of type A and those of type B to which the electromechanical failing load test ( see 10. as regards their essential dimensions.7.7.1.2 For insulators with ball and socket couplings.
*Specification for insulator fittings for overhead power lines of 3. as regards their essential dimensions.4 The insulator passes the test if the specified mechanical failing load is reached. the coupling pieces of the testing machine shall be in accordance with IS : 2486 (Part II)1974*. 10.1.1. 10.12 is reached and the value recorded.8.7 To provide information when specially requested. the insulators shall be submitted to the mechanical failing load test ( see 10.1.8 ).7. 10. the coupling pieces of the testing machine shall be in accordance with IS : 2486 (Part II)1974*. and it shall be maintained at this value throughout the test. without puncture. 10.7 ) is not applicable shall be subjected individually to a tensile load applied between the metal parts.7. 10. 10.5 The tensile load shall be gradually increased from a value of 75 percent of the specified electromechanical failing load until the specified electromechanical failing load is reached.8.7.2 The insulator units shall be subjected individually to a power-frequency voltage and to a tensile load applied simultaneously between the metal parts.7 Electromechanical Failing Load Test (on Type B String Insulator Units Only) 10.3 kV and above: Part II Dimensional requirements ( first revision ).
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.8.4 The voltage shall be 50 percent to 60 percent of the specified wet power-frequency withstand voltage of the string insulator unit.7. For other types.3 For insulators with ball and socket couplings.1 String Insulator Units 10. the load may be increased until the failing load as defined in 2. 10.8.6 The insulator passes the test if the specified electromechanical failing load is reached.

10.13 is reached and the value recorded.1 The insulator shall be subjected for 24 hours to a tensile load.13 is reached and the value recorded.5 To provide information when specially requested. after having been cleaned and dried.10.15.10. 10.8. The insulator shall be subjected to a load equal to 75 percent of the specified mechanical failing load applied perpendicular to the axis of the insulator in the plane of the side groove by means of a wire rope encircling the side groove. shall be completely immersed in a tank containing a suitable insulating medium to prevent surface discharges on them. the load shall be applied to this clamp. by agreement between manufacturer and purchaser.1.9.3 To provide information when specially requested. insulators provided with integral metal fittings for mounting shall be mounted for test using these fittings. 10.10. 10.8.1.2 The insulator shall then pass the power-frequency routine test given in 10.8.8.2. The wire rope shall be such that lacalized stresses in the side groove of the insulator are avoided.1 Power-Frequency Puncture Test 10. 10.1 Rigid insulators shall be mounted on a rigidly fixed pin capable of withstanding without appreciable deformation the loads to which it will be subjected during the test.0 The puncture test may be either a power-frequency puncture test or. 10.2 The insulator passes the test if the specified mechanical failing load is reached.1 The insulators. If the insulator is provided with means of clamping the conductor.2.2.9. applied axially which shall be two-thirds of the specified minimum failing load. If the tank is made of metal. the load may be increased until the failing load as defined in 2.1971
10. an impulse overvoltage test.10 Puncture Test (for Insulators Type B Only) 10. The load shall be gradually increased until the specified mechanical failing load is reached.IS : 731 . its dimensions shall be such that the shortest distance between any part of
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.8. 10.2 Rigid Insulators 10. the load may be increased until the failing load as difined in 2.9 Twenty-four Hours Mechanical Test (for String Insulator Units only when Specified by Purchaser) 10.

11.11 Porosity Test 10.11.1 Porcelain fragments from the insulators or.1971
the insulator and the side of the tank is not less than 1.10.1.1 If this test is required.11. 10.2 Impulse Overvoltage Test 10. by agreement. 10.12.10. washed dried and again broken. 10.2 The test voltage shall be applied between those parts which normally have the operating voltage between them.12 Galvanizing Test 10.3 The voltage shall be raised as rapidly as is consistent with its value being indicated by the measuring instrument to the specified puncture voltage.1 This test comprises firstly verification of the uniformity of the coating of zinc (test by immersion in copper sulphate) and secondly verification of the weight of zinc per unit surface (test by chemical dissolution).1.10. 10.5 times the diameter of the largest insulator shed. No puncture shall occur below the specified puncture voltage.12. but one desirable property is a slight conductivity (resistivity of the order of 106 – 108 Ωm)
10. the voltage may then be raised until puncture occurs and the puncture voltage is recorded.3 Examination with the naked eye of the freshly broken surfaces shall not reveal any dye penetration.
*Methods of testing uniformity of coating on zinc coated articles ( first revision ).10.2.4 To provide information when specially requested. Penetration into small cracks formed during the initial breaking shall be neglected.1. During immersion in the insulating medium.2 The fragments shall then be removed from the solution. 10. 10.
NOTE — It is not possible to define exactly the properties of the immersion medium. from representative pieces of porcelain fired adjacent to them shall be immersed in a 1 percent alcohol solution of fuchsin (1 g fuchsin in 100 g methylated spirit) under a pressure of not less than 15 × 106 N/m2 for a time such that the products of the test duration in hours and the test pressure in N/m2 is not less than 180 × 106. The immersion medium shall be at about room temperature. the details shall be agreed to between the purchaser and the manufacturer.2 The uniformity of zinc coating of galvanized metal fittings shall satisfy the requirements given in IS : 2633-1972*. 10. 10. precautions shall be taken to avoid air pockets under the sheds of the insulator.10.IS : 731 .
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. This second test is optional and shall be carried out only if so agreed by the manufacturer and the purchaser.

10.cm 2 000
Also the area of any single glaze defect shall not exceed:
2 DF 0. the total area not covered by glaze shall not exceed:
2 DF 1 + -------------.
NOTE — Mechanical routine tests on Type A rigid insulators shall be made only by agreement between purchaser and manufacturer.1 A visual examination shall be made on each insulator. the voltage shall be applied between the metal parts.1 Type B string insulator units and rigid insulators shall be subjected to a power-frequency voltage.13 Visual Examination Test 10.14. in centimetres.
10. 10.15.14.2 Type B string insulator units shall be subjected for at least 10 seconds to a tensile load equal to 40 percent of the specified electro-mechanical failing load.
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.1971
10.3 Insulators which break or whose metal parts are fractured or become detached during the test shall be rejected. in centimetres.14.13.15 Electrical Routine Test (for Type B Insulators) 10. but the normal metal work of interlinked insulators may be replaced by other suitable fittings. Some variation in the colour shade is permitted and shall not justify rejection of the insulator. The insulator shall be free from physical distortion of shape within tolerances specified. The colour of the insulator shall approximate to the colour specified on the drawing. 10.1 Type A string insulator units shall be subjected for at least 1 minute to a tensile load equal to 60 percent of the specified mechanical failing load. The areas specified as glazed on the drawing shall be covered by a smooth hard glaze free from cracks and other defects prejudicial to satisfactory performance in service. F is the creepage distance of the insulator.14 Mechanical Routine Test (for String Insulator Units only) 10.5 + ----------------. For string insulator units.IS : 731 .cm 20 000
where D is the greatest diameter of the insulator. Out of those areas specified as glazed on the drawing.

10. Alternatively.
APPENDIX A
( Clauses 3.289p d = -----------------. metal electrodes may be used provided the electric stress in the porcelain is not reduced. 10. 10.
A-2.1. GENERAL
A-1.
NOTE 1 — This test should preferably be carried out after the mechanical routine test. NOTE 2 — For certain designs of rigid insulator Type B.15. 10. and the voltage shall be applied between the tank and water practically filling the pin hole or cavity of each insulator. but it shall not disqualify the insulator for acceptance.4. the test on the assembled insulator may then be replaced by a test on the insulating parts before assembly. are not significantly affected by these changes.5.(where d lies between 0.3 The test voltage shall be such as to produce frequent flashover (every few seconds).4.05) 273 + t
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.2.5 ) CORRECTION OF TEST VOLTAGES FOR ATMOSPHERIC CONDITIONS A-1.1971
10. however. 10. it may not be possible to apply the test described above. adjustments should be made to certain of the test voltages by the application of the following correction factors in accordance with Table 4: a) Correction factor for air density d: 0.IS : 731 . CORRECTION FACTORS
A-2.15.15.2 Rigid insulators shall be placed head downwards in a tank containing water to a depth sufficient to cover the side conductor grooves.1 When the atmospheric conditions in the neighbourhood of the insulator during the test differ from the reference conditions.1 Variations in barometric pressure and in humidity of the atmosphere cause variation in the electric strength of the air and hence also in the withstand and flashover voltage of insulators exposed to the air. in order to eliminate insulators which may have been partially damaged in the mechanical test. By agreement between the purchaser and the manufacturer. Streamer pattern formed during routine electrical tests may be observable.3. under-oil flashover and puncture strength.3 and 10.2.95 to 1. 10.15.3.5 Insulators which puncture during the test shall be rejected.4 The time of application of the test voltage shall be at least 5 consecutive minutes.

3A corresponding to the observed value of the difference of dry and wet bulb temperatures. 3B. Read the correction to be applied to humidity from the right hand side of Fig.IS : 731 . This correction is positive for a positive deviation and negative for a negative deviation.
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. from the standard atmospheric pressure. 2
DETERMINATION OF ABSOLUTE HUMIDITY
Locate the point corresponding to the deviation of ambient atmospheric pressure from 1 013 millibars on the left hand side of Fig. Then locate the point on the curve in Fig. 3B corresponding to the point of intersection. Draw a vertical line through this point to intersect the straight line drawn in Fig. 3B and join it with right hand side top corner by a straight line.1971
FIG.

1 The insulator shall be mounted on an earthed metallic vertical pin. No other object shall be nearer to the insulator than 1 metre or 1.1 and 10.3.1.1.2.1. with a smooth surface and a diameter not less than 16 mm. B-2.4 Precautions shall be taken to avoid flashover from the ends of the conductor. the sample insulators shall be clean and dry and in thermal equilibrium with its surroundings (ambient medium). CONDITION OF THE INSULATORS
B-1.3 The length of the conductor shall be at least 1. B-2.5 times the diameter of the lowest insulator. whichever is the greater.5 times the length of the insulator string.2 Rigid Insulator B-2. and the distance from the lowest shed of a porcelain part to the upper surface of the conductor shall be as short as possible but greater than 0.
B-2. and it shall extend at least 1 metre on each side of the vertical axis. The distance between the uppermost Point of the insulator metal work and the supporting structure shall be not less than 1 metre. whichever is the greater. B-2.5 times the height of the insulator.1 String Insulator Unit and Insulator String B-2. 10.1971
APPENDIX B
( Clauses 10.5 percent of the length of the string insulator unit or insulator string with a minimum of 25 mm.
27
.1 ) HIGH VOLTAGE TESTS B-1.1. A length of conductor in the form of a straight smooth metal rod or tube shall be attached to the lower integral fitting of the string insulator unit or insulator string so that it lies in a horizontal plane.5 The test voltage shall be applied between the conductor and earth. B-2.4.2 The diameter of the conductor shall be about 1. METHODS OF SUPPORT
B-2.1 Before commencing the tests. No other object shall be nearer to the insulator than 1 metre or 1.IS : 731 .1.5 times that of the string insulator unit or insulator string.2. B-2.1 The string insulator unit or insulator string shall be suspended vertically by means of an earthed wire rope or other suitable conductor from a supporting structure. The pin shall be sufficiently long to ensure that the lowest edge of the insulator is at least 1 metre above the ground.

2.2 The standard 1. Except by agreement between the manufacturer and the purchaser.2 A conductor not less than 5 mm diameter and extending in both directions at least twice the insulator height beyond the top shed shall be secured as nearly as possible horizontally in the side groove of the insulator by means of a metallic wire of about 1 mm diameter wrapped round the conductor for a distance approximately twice the diameter of the top shed and extending equally on each side of the insulator.
B-4. the insulator shall be maintained at the ambient temperature of the test location for sufficient time for equilibrium to be reached before the test commences.1 The impulse tests shall be made in accordance with IS : 2071 (Parts I and II)-1974*. B-2. Part II Test procedures ( first revision ). IMPULSE TESTS
B-4.
NOTE — When so agreed.2/50 impulse wave shall be used.1 The time intervals between consecutive flashovers shall be sufficient to avoid effects from the previous application of voltage in flashover or withstand tests.2 The test circuit shall be such that when the test object is short-circuited at the test voltage.
B-5. the current is not less than 0.5 A if wet tests are to be made.1 A if dry tests are to be made. and not less than 0.IS : 731 . B-4. the characteristics may differ from the values measured using the standard methods of mounting.1 The power-frequency tests shall be made in accordance with IS : 2071 (Parts I and II)-1974*. B-5. POWER-FREQUENCY TESTS
B-5. taking account of all factors which may influence flashover voltage. tests shall not be made if the relative humidity exceeds 85 percent.2. For example.3 If the rigid insulator is provided with means of clamping. EXCESSIVE HUMIDITY
B-6.1 Precautions shall be taken to avoid condensation on the surface of the insulator. conductor shall be placed in the clamp.1971
B-2. especially when the relative humidity is high.
B-3.
*Methods of high voltage testing: Part I General definitions and test requirements ( first revision ). tests may also be made under conditions reproducing service conditions as closely as possible.2. Under these conditions. B-2.
B-6.
28
.4 The test voltage shall be applied between the conductor and earth. TIME INTERVALS BETWEEN FLASHOVERS
B-3.

a second sample of the same size ( see col 3 of Table 5 ) shall be selected from the lot at random and subjected to these tests.2. If there is one defective. Otherwise the lot shall be rejected without further testing.
C-2.IS : 731 . a second sample of same size ( see col 8 of Table 5 ) shall be selected at random and subjected to the
*Methods for random sampling
29
. C-1. SCALE OF SAMPLING
C-1.3 ) SAMPLING PROCEDURE FOR INSULATORS FOR OVERHEAD POWER LINES C-1.2.1 and 10. procedure given in IS : 4905-1968* may be followed.2 The number of insulators to be selected from each lot shall depend upon the size of the lot and shall be in accordance with col 1 and 2 of Table 5 and procedure given in C-2. In order to ensure the randomness of selection. The number of insulators to be tested for mechanical. The lot shall be considered as conforming to these requirements if no defective is found in the sample and shall be rejected if there are two or more defectives.2 The lot which has been found as conforming to the above requirements shall then be divided into two parts.1 These insulators shall be selected from the lot at random. The insulators failing to satisfy either of the requirements shall be termed as defectives.1.1 Lot — In a consignment.1971
APPENDIX C
( Clauses 10. The lot shall be rejected if the number of defectives in the same lot is greater than or equal to the first rejection number ( r1 ) given in col 5. all the insulators of the same type and design manufactured from the same material in the same factory under similar conditions of production shall be grouped together to constitute a lot. The number of defectives in the first sample and second sample shall be combined. as shown in col 7 and 9 of Table 5. The lot shall be considered as conforming to these requirements if the number of defectives found in the sample is less than or equal to corresponding acceptance number given in col 4 of Table 5.2.1. electro-mechanical and porosity tests shall be in accordance with col 7 of Table 5. the lot shall be considered as conforming to these requirements. If the number of defectives is between the acceptance number and the first rejection number.1 All the insulators selected at random according to col 1 and 2 of Table 5 shall be subjected to dimensions and temperature cycle tests. NUMBER OF TESTS AND CRITERIA FOR CONFORMITY
C-2. C-1. If the combined number of defectives is less than the second rejection number( r2 ) given in col 6 of Table 5. C-2.

IS : 731 . C-2. a second sample of same size ( see col 10 of Table 5 ) shall be selected at random and subected to the tests. For this purpose.
31
.1. otherwise the lot shall be rejected without further testing.1 shall then be tested for galvanizing test and puncture test. C-2.4 The lot shall be considered as conforming to the requirements of acceptance tests if conditions in C-2.3 are satisfied. C-2.1971
tests. The lot shall be considered as conforming to these requirements if no defective is found in the sample and shall be rejected if two or more defectives are found in the sample. otherwise the lot shall be rejected without further testing.3 The lot which has been found as conforming to the requirements of C-2. If there is one defective. The lot shall be considered as conforming to these requirements if no defective is found in the second sample.2 and C-2. the sample size is given in col 9 of Table 5. The lot shall be considered as conforming to these requirements if no defective is found in the second sample.

1 Q)] – Substitute ‘Mechanical Performance Test (10.2)’ for ’24 hours mechanical strength test {for string insulators only when specified by the purchaser) (10. clause 10. (Page 19.
.9. after loading and unloading. the insulator units shall be subjected to a tensile load equal to 60 percent of the specified electromechanical or mechanical failing load (whichever is applicable). During the initial stage of the test.9. On the same day.1 Mechanical for the existing text:
P@ivm7nce Test
This test has an initia[ stage of mechanical loading and unloading.9.9) — Substitute ‘Mechanical Performance Test and Thermal Mechanical Performance Test’ JOY ‘Twenty-four Hours Mechanical Test (for String Insulator Units only when Specified by Purchaser)’.9. (Page 19.NO. and a concluding stage of testing the insulator units to failure.1. c/ause 10. clause 10. the insulator units shall be subjected individually to an electromechanical or mechanical failing load test. The concluding stage is identical to an ordinary electromechanical or mechanical fail ing load test.1) — Substitute the following 10. The performance of the insulator units will be determined by a comparison of the failing load values and the fracture pattern obtained during the electromechanical and mechanical failing load test and the same test carried out as the final stage of the mechanical performance test defined here. the tensile load shall be applied and immediately removed four times in succession. Unless otherwise agreed. 7 JIJLY 2008 TO IS731 :1971 SPECIFICATION FOR PORCELAIN INSULATORS FOR OVERHEAD POWER LINES WITH A NOMINAL VOLTAGE GREATER THAN 1000 V
(Second Revision )
AMENDMENT
[Page 13.9)’.1) and l-henna] Mechanical Performance Test (only on string insulator units) (10.

Unless otherwise agreed.3: Performance Test
10.9. and a tensile load equal to 60 percent of the specified electro-mechanical or mechanical failing load.9.!e 10. on cotnpletion oft he fourth 24-hour cycle and coolinp to room tmnperature. 2 The insulator units may be coupled together in series andlor in parallel when subjected to the 60 percent load.2 Therma[-Mechanical
During the initial stage of the test.9. then heating. after thl~ Iond removal. h is possible to investigate the fundamental insulfllor design by using metal parts suitably IIlmlatm
III
strengthened so that failure
occurs
in the internally
stressed zone of the
ccm!tiolls
shollkl
be Itrken so that strengthening
tbe metal p[irts dues not affect the
I’llndamental strew relaliun.3 ‘Ile tes~ may not give information on the internally stressed zone if [he failure occurs in a mctid part such as the socket or clevis of a cflp or an insulator pin III nucb cases. c/m~.2
(f’age 19.9. /ast line) -– Insert the folhmdllg ancl renumber ilS 10. but by agreement it may be used ari a (design test or special qualificatmn test. The temperature figure refers to the surrounding air. each 24-hour cycle shall comprise ~ cooling to – 30 + 5°C and a heating to + 40 + 5“C.1. the 2
. The temperature sequence shall be first cooling. 4 Voltage may bc applied to the insulator units (Type B only) whenever II is desired in the course of the test 10 detect mechanical failure of the insulating part (punclurc). The tensile load shall be applied to the insulator units at room temperature before starting the first thermal cycle.Amend No. 7 to 1S 731:1971
NOTES 1 ‘llc test may be more decisive if the 60 percent load is applied and removed more than four times in succession. “l’he luusi]e load shall be completely removed and re-applied towards the end of ~i~ch I KM ing period and then last one accepted.
I (). tllc insulator units must he equally loaded. The test equipment shall be such as to permit the minimum and maximum tetnperatures each to be kept during atleast four consecutive hours of the tcmpomture cycle. On the same day. When parallel coupled. . this teSl iS uot suitable for R sample test. the insulator units shall be subjected to four 24-hours cycles of cooling and heating. the tensile load shall be removed.

14. New Delhi. tlw metal parts does not affect the
should be taken so that strengthening
fundamental stress relation. In such oases. it is possible M investigate the fundamtmtal insulator design by ttaing metsrl parts sullahly strengthened so that Iailure occurs In Precautions
the internally stressed zone of’ the
instdtrtor.
2 The insulalot units may be coupled together in series a!ldor in parallel when subjected to the thermal cycles and the 60 percent load. and should not be repetrted on typtw \vhich differ In outward ~orm Onlyj that Is. but withotd altering the test cycle. c/uuse 10. Changes in hlternal design or in manufacturing processes are reasons Ibr re-testing. in ordet hI detect insulating prut (puncture). 7 to IS 731:1971 insulator unit shall be subjected rnechstnical failing load test. for example. NOTES I This thermal mechanical performance test has reference to the fundamental insulcrtor desig!! in respect of Ihe internal stresses. The tesl may not givr information on the internally sl~essed zone if the ftril!m) occurs in a metal parl such as the Rocket rrr clevis of a cap or an insulator pin. those used with irwulntors of tlw long rod type. Indii
3
. should not be included in the mechanical test since they are a! ~{ part of (Iw internal design of thu insulator. Ihc insulator units must be equally Ioadetl.2) – Substitute ’50 percent of the specified electromechanical fail ing load’ @r ’40 percent of the specified electro-mechanical failing load’. When parallel coupled.’ Itwchtmical failure of the
(Page 21.
Ftul her the tolemnces on the temperatures of the hot and (xlld cycles shall b(! respected in such a way so as to ensure a minimum difference of 70 degrees between the recorded hot and cold temperatures. the disc of the insulating part or the coupling ends of the metal fittings. individually IL) an electromechanical m
The performance of the insulator units will be determined I)y a comparison of the failing load valtles and the fracture pattern ol)lllined during the elect romechanicttl and mechanical failing load test Ilnd the ~~alnetest carried OLII as tile final stage c]f the mechanical performance test definefl IIere.Amend No.
(ET 06)
~prography Utfi-: BIS. 4 Voltage may be applied to the insulator units (Type B only) wherwvrr desired in the course of the test. 3 Loose coupling pins.